A power supply or a voltage regulator (VR) is widely used in various electronic systems to provide a constant voltage level and a current required by a load. Moreover, modern electronic system may include components that maintain current requirements across a broad range, from relatively high peak currents to very low currents in light load operation mode. In all the power supply and converter designs, one of key techniques is how to accurately and timely acquire current signal that will be used for current control and monitoring purpose.
For a hypothetical server system for example, an ever increasing accuracy of current-sense (CS) and power telemetry data are required to be used in system level power and thermal management algorithms. For example, the central processing unit (CPU) power management algorithms need load current information from the CPU Voltage Regulator. CPU load current information is used to optimize CPU performance versus Running Average Power Limit (RAPL) control while still maintaining safe operation of the system. Accuracy of current sense directly impacts system performance, reliability, and power saving. Therefore, tighter current-sense accuracy targets are expected for a next generation of servers. ±2% current-sense accuracy specification may be a future trend for a wide load operation range. However, attaining such accuracy to improve system performance using generally-known techniques introduce either extra power loss or additional cost for a power processing unit in an electronic system.
Conventionally, a relatively accurate current-sense can be achieved by using a dedicated sense resistor in series to the output inductor. However this traditional method would introduce quite a bit of extra power loss, especially for high output current applications.
To minimize unnecessary power loss on current sensing, an inductor direct current resistance (DCR) sensing scheme is widely adopted in the industry. Inductor DCR Current Sensing uses the voltage drop on the output inductor DC Resistance (DCR) to derive the output current information. Although the DCR based sensing scheme is lossless, it's difficult to meet the ever increasing current-sense accuracy requirement for existing and future applications. The DCR-based current sense accuracy is also impacted by DCR tolerance, time constant mismatch, printed circuit board (PCB) assembly process and undesired temperature compensation.
An alternative way may be to use MOSFET RDS (on) based current sense technique. The metal oxide-semiconductor-field-effect-transistors (MOSFET) is a critical component being widely used in a power converter or a VR, and RDS (on) is referred to as drain-to-source resistance in an on-state for a MOSFET device. RDS (on) based current sense method has been widely adopted since it does not need extra sense components and external time constant match. However, the RDS (on) based current sense accuracy is significantly impacted by the tolerance (usually 20-40%) of the MOSFET on-state resistance (RDS (ON)) from a MOSFET component to another, and a difficult temperature compensation implementation.
Another alternative way may be to use the “mirror FET” or “pilot device” to proportionally sense the current flowing through a main power device such as a MOSFET. An accurate current-sense in a full load operation range may be potentially achieved by pilot device based current sense method. Current sense accuracy will no longer be impacted by silicon process variation, and temperature compensation will no longer be needed since the pilot device is from same, one die; and thus, it should best represent operation of the main power device. Pilot device based current sense method is theoretically the best approach suited to modern monolithic design of power MOSFET.
For the monolithic power MOSFET, for example, in a synchronous Buck converter or the power stage, the power MOSFET is laid out as the millions of cells connected in parallel by the metal layers. The current flowing through the main MOSFET is sensed by the pilot device that may be one cell or several cells of the same main device, which is normally located at the center of the main MOSFET. The pilot device has the same gate and source voltage as that of the main device; the drain of the pilot device enforces a current to make the same drain voltage as that of the main MOSFET. Therefore, the current flowing through the pilot device proportionally represents the current flowing through the main field effect transistor (FET). The current ratio of the pilot device versus the main MOSFET device is proportional to the geometric ratio of pilot device versus the main device. This is true for the small size, low power MOSFET in which the main power MOSFET has the uniform current distribution.
However, for the high-current high-power application, the MOSFET area become so big. As such, the current flow is no longer uniform at the different location of the silicon due to non-identical metal resistance of each cell of the main MOSFET and the current flowing direction on a specific application PCB. The current non-uniformity also depends on the local temperature of the cells in the silicon. Current sense accuracy has to be compromised if the pilot device is placed at the one location, for an example of center of main the MOSFET.
This invention proposes a method to use multi-cell pilot devices and put the pilot device at the different locations of the main MOSFET. So the big size main power MOSFET is separated into many sections and each section has its own pilot device to match the location and temperature. In the meantime, in order to deal with the multi-pilot devices, a multi-input amplifier stage is invented as a pre-stage for the amplifier to sum the current information of the multi-cell pilot devices.
In one or more embodiment, the present innovation provides a current sense scheme for MOSFET RDS (on) sense based voltage regulator (VR). Same concept or method may be used in a power stage device or power processing unit in an electronic system.